The present invention provides a low temperature process for preparing an aromatic acid by reacting an aromatic aldehyde with air at a temperature of about 20xc2x0 C. to less than 100xc2x0 C. in the presence of a supported Group VIII metal catalyst.
4-Isopropylbenzoic acid (cumic acid or cuminic acid), which is an aromatic mono-carboxylic acid, is an important organic intermediate in the production of pharmaceutical compounds. For example, 4-isopropylbenzoic acid can be used to synthesize nateglinide, a compound for treating diabetes available under the trademark STARLIX from Novartis. Several methods have been used to prepare isopropylbenzoic acid from isopropylbenzaldehyde. Such methods typically employ deleterious oxidizing agents, such as peroxides, NaOCl, peracids, NaIO4, and O3 which can be used with or without a catalyst. The use of such oxidizing agents, however, pose safety concerns because they are corrosive, and disposing of such oxidizing agents may cause environmental problems, especially the by-product salts such as halogen-containing oxidants.
U.S. Pat. No. 3,946,067 describes a process for preparing benzaldehyde by vapor phase oxidation of toluene at a temperature from 100xc2x0 C. to 250xc2x0 C. in the presence of a catalyst composition containing phosphoric acid and a palladium metal. U.S. Pat. No. 5,693,856 describes a process for preparing terephthalic acid which involves reacting a polyalkyl benzene compound with oxygen in the presence of a heavy metal catalyst at a temperature from 150xc2x0 C. to 250xc2x0 C. U.S. Pat. No. 5,686,638 describes a process for preparing mono- or dicarboxylic acids from an aldehyde without using a catalyst or an aprotic solvent, which involves oxidizing the aldehyde in a carboxylic acid as diluent or carboxylic acid/water mixture.
U.S. Pat. No. 5,189,209 describes a process for preparing highly pure isophthalic acid which involves oxidizing m-dialkyl benzene with molecular oxygen in an acetic acid solvent in the presence of a heavy metal catalyst at a temperature of 100xc2x0 C. to 240xc2x0 C. to form crude isophthalic acid; and contacting the crude isophthalic acid with a Group VIII catalyst supported by activated carbon in the presence of hydrogen at a temperature of 170xc2x0 C. to 300xc2x0 C. U.S. Pat. No. 5,110,982 describes a process for producing 2,6-naphthalene dicarboxylic acid which involves oxidizing a 2-alkyl-6-acyl naphthalene with molecular oxygen-containing gas in the presence of a catalyst containing cobalt, manganese, and bromine in an acetic acid solvent. U.S. Pat. No. 4,603,220 describes a process for preparing aromatic monocarboxylic acids by oxidation of a liquid phase of toluene or substituted toluenes with oxygen or gas containing oxygen, and a heavy metal catalyst.
Japanese Patent No. 3038100 describes a process for preparing aromatic carboxylic acids by oxidation of aromatic aldehydes with a peracid. Japanese Patent Application No. 87-99169 describes preparing cuminic acid by oxidation of cuminaldehyde with hydrogen peroxide under alkaline conditions.
Air oxidation of 3,4-dimethoxybenzaldehyde to 3,4-dimethoxybenzoic acid using cobalt acetate, manganese acetate, or lithium bromide as catalysts and a reaction temperature of from 110xc2x0 C. to 140xc2x0 C. is described by Mukhopadhyay, Organic Process Research and Development, Vol. 3, pp. 365-369 (1999).
Therefore, it is desirable from a safety and environmental standpoint to develop a process for preparing an aromatic acid from an aromatic aldehyde that (i) does not involve the use of deleterious oxidizing agents; (ii) is essentially free of by-products; and (iii) is accomplished at a temperature of less than 100xc2x0 C.
The invention provides a low temperature process for preparing an aromatic acid having formula (I), 
said process comprising reacting an aromatic aldehyde having formula (II) 
with a gas having an oxygen content of 1 to 100 weight percent, based on the total weight of the gas, at a temperature of about 20xc2x0 C. to less than 100xc2x0 C. in the presence of a supported Group VIII metal catalyst, and a solvent having a flash point greater than 95xc2x0 C. and/or a melting point less than 55xc2x0 C., provided that the flash point of the solvent is greater than the reaction temperature, wherein R1 and R2 are independently selected from the group consisting of hydrogen and a C1-C8 linear, branched or cyclic alkyl group. Preferably the aromatic acid is 4-isopropylbenzoic acid and the aromatic aldehyde is 4-isopropylbenzaldehyde.
The process of the invention yields an aromatic acid in excellent yield, and (i) does not involve the use of deleterious oxidizing agents such as peroxides, NaOCl, peracids, NaIO4, and O3; (ii) is essentially free of by-products; and (iii) is accomplished at a temperature of less than 100xc2x0 C.